The present invention relates to an etching method and apparatus in which a pattern formed on the surface of an object to-be-etched is directly monitored, whereby the etching is automatically controlled in-process.
Lithography for forming the patterns of an IC and an LSI includes the so-called wet etching process which employs chemicals, and dry etching process which resorts to a reactive gas in a plasma. The dry etching process is excellent in the dimensional accuracy of the etched pattern, and forms the mainstream technique of the LSI and VSLI. On the other hand, however, the ratio between the etching rates of a photoresist serving as a mask and a desired material to-be-etched or another material underlying the desired material to-be-etched is as small as 2-15 when compared with about 50 in the case of the wet etching process. It is therefore important to properly and accurately detect the end point of the etching, to stop the etching and to prevent the progress of surplus etching. If the detection of the end point of the etching is inaccurate, the etching is incomplete. Conversely, if the etching period of time becomes longer than an appropriate value, the photoresist being the etching mask and/or a layer underlying the desired material to-be-etched can become undesirably corroded.
As methods for detecting the end point of the etching with the plasma, there are (1) the emission spectrographic method which detects light emitted by a seed electrically excited by glow discharge, an etching product or a reactive etching seed, (2) the light reflection method in which, by utilizing the change of the reflection factor of a material under etching or the interference effect of light of a thin film under etching, the reflection factor of light of the material under etching is measured as a function of time and the thickness of the film is monitored, (3) the method in which a seed in a gas stream introduced into an etching reactor or a reaction product substance is sampled and is analyzed mass-spectrographically so as to sense the change of a signal with the progress of etching, (4) the method which measures the change of a plasma impedance owing to fluctuations in the concentration of a reaction seed and the concentration of an etching product that change with the progress of etching, (5) the method in which the density of electrons or the distribution of electronic and ionic energies fluctuating due to the change of the composition of a plasma attendant upon the progress of etching is observed by the use of a Langmuir probe, and (6) the method which measures the fluctuations of a pressure in an etching vessel changing with the progress of etching. The known methods stated above are described in detail in "Methods of Detecting End Points of Plasma Etching" (July 1981/Solid State Technology/Japanese Version, page 62) by Paul J. Marcoux Pang Dow Foo.
Regarding the emission spectrographic method which is used most extensively among the techniques listed above, the problems of the prior art will be described with reference to FIG. 1.
An etching processing chamber 1, in which an upper electrode 2 and a lower electrode 3 are installed, is provided with a lighting window 6 for taking out light emission based on the discharge plasma between the electrodes. A lens 7, a monochrometer 8 and a photomultiplier 9 are disposed on an identical optic axis outside the lighting window 6. The output of the photomultiplier 9 is amplified by an amplifier 10, and then sent to an etching end point decision device 11.
A gas is introduced into the etching processing chamber 1 from an etching gas feed device, not shown, and the chamber is evacuated by a vacuum pump, not shown, whereby the interior of the processing chamber is held at a fixed pressure (0.5-50 Pa). When a high frequency voltage is applied across the upper electrode 2 and lower electrode 3 from a radio frequency power source 5, glow discharge arises across the electrodes, and the etching of a wafer 4 on the lower electrode 3 is started. The emission based on the plasma discharge is fed into the monochrometer 8 through the lighting window 6, to take out from within the emission of the discharge only the light of a wavelength which changes with a correlation to the progress of the etching. For example, in the etching of Al, the light of a wavelength of 396 nm which is one of the emission spectrum of Al molecules is separated by the monochrometer 8, and the intensity variation thereof is measured by the photomultiplier tube 9. Then, output changes which correspond to the situations of the starting of the discharge and the starting and end of the etching of the Al are measured. The output signal is electrically processed by the end point decision device 11, to detect the end point of time of the etching.
With the emission spectrographic method, changes in the intensity of the plasma emission arise due to pressure fluctuations in the etching processing chamber and output fluctuations of the radio frequency power source. It is therefore difficult to accurately decide the point of time of the completion of the etching. Moreover, as illustrated in FIG. 2, the change of the output signal (indicated by current) of the photomultiplier tube near the point of time of the completion of the etching is not abrupt though great in the changing rate, so that a dispersion in the decisions occurs. In FIG. 2, A denotes the starting time of the discharge, B the starting time of the etching, and C the end time of the etching.
This method detects the reaction seed or etching product which exists between the electrodes, and needs to directly and accurately obtain the correlation with the etched state of the wafer surface in advance. It involves such problems that the correlation is affected by the flow of the etching gas or an organic reaction product adhering to the wall of the processing chamber or the electrode, and that the correlation slightly differs depending upon the lighting range. It has accordingly been difficult to detect the etching end point at high precision.
Meanwhile, as disclosed in Japanese Laid-open patent application No. 53-138943, the light reflection method is the method in which the surface state of the material to be etched is directly observed, and hence, it is free from the indistinctness as involved in the emission spectrographic method stated above. In LSI wafers etc., however, the reflection factors of the surfaces of the materials to be etched differ depending upon lots, and a certain degree of dispersion (e.g., 20% in case of Al) is included. Moreover, the method is affected by the change of the emission intensity ascribable to the instability of the plasma discharge and is difficult to detect only the variation of the reflection factor, so that the S/N (signal-to-noise) ratio worsens. With the method which utilizes the interference of light on the surface of the material to-be-etched employing a laser beam, signals to intensify the light and signals to weaken it appear several times with the progress of etching. Therefore, the end point detection algorithm becomes complicated, and the decision is liable to err. Besides, in a case where a material of good light transmission is concerned or where the reflection at the boundary between the underlying material and the substance to-be-etched is intense, a comparatively favorable result is obtained, but in any other case, the signal which is obtained is generally inferior in the S/N ratio. Another disadvantage is that, when a fine pattern exists on the surface of the material to-be-etched, a signal of good S/N ratio is not obtained.